The Sky This Week, 2017 March 28 - April 4

The springtime's brightest star

Moon and Mercury at dusk, imaged 2016 April 8
from the U.S. Naval Observatory, Washington, DC
with a Canon EOS Rebel T2i DSLR

The Moon returns to the evening sky this week, waxing to the First Quarter phase, which will occur on April 3rd at 2:39 pm Eastern Daylight Time. Luna’s crescent may be found nine degrees to the south of the fleet planet Mercury during deepening twilight on the evening of the 29th. On the 30th she is seven degrees south of ruddy Mars. As April opens she moves serenely through the stars of the Great Winter Circle.

Winter’s constellations are now rapidly giving way to the rising stars of spring. You’ll find all of the bright stars of the Great Winter Circle west of the meridian at the end of evening twilight, and they begin to set with the disappearance of Aldebaran at local midnight. That said, you still have some time to explore the bright colorful stars of Orion and his surrounding companions before the more subdued stars of spring work their way into prominence. You’ll appreciate this as the waxing Moon begins to wash out the fainter stars in the sky by the end of the week. The area surrounding Orion contains nine of the 25 brightest stars in the sky, while east of the meridian the pickings are definitely slimmer. Here you’ll find only three first-magnitude stars spread over a wide swath of the sky. The brightest of these, which you can spot low in the eastern sky at 9:00 pm, is Arcturus, lead star of the constellation Boötes the Herdsman. As the star climbs higher you can appreciate its pale orange tint and prominence. It is the third-brightest star in the sky and the brightest in the northern celestial hemisphere. Its color arises from its status as an evolved star that has depleted much of its supply of fusible hydrogen, causing it to swell into a so-called "giant" phase. It is also a star located in a diffuse "halo" of older stars that surrounds the Milky Way, and its motion through space carries it almost perpendicular through the plane of the Galaxy is the most rapid of all of the first-magnitude stars. Since the time of Ptolemy it has moved about a degree with respect to more distant background stars!

The elusive planet Mercury is now at the peak of visibility for this, his best evening apparition of the year. He reaches his greatest elongation from the Sun on April 1st, when he lies 19 degrees east of the Sun. Start looking for him low in the deepening evening twilight about half an hour after sunset; he’ll be about 10 degrees above the west horizon. On the evening of March 30th he’ll be nine degrees to the right of the slender crescent Moon. If you can get the telescope trained on him he’ll resemble a tiny first-quarter phase Moon.

Mars is also visited by the Moon on the evening of the 30th. The red planet will lie about seven degrees to the right of the Moon’s crescent. Mars continues his trek across the stars of Aries, drawing a bead on the brighter stars of nearby Taurus.

Jupiter continues to press toward opposition, which will be upon us in just over a week. The giant planet rises in the eastern sky at around 8:00 pm this week, and can be seen in the company of the bright star Spica as he traverses the overnight hours. The planet’s ample-sized disc reveals fine details in almost any telescope, but if you have a four-inch or larger instrument look for the famous Great Red Spot crossing the disc on the night of April 2nd. Any instrument will show the planet’s four large Galilean moons.

Saturn waits patiently for his turn in the evening sky. This week he is slowly drifting through the stars of Sagittarius, which is best seen just before the first rays of morning twilight. His far southerly declination hampers detailed views of his subtle cloud patterns, but you should have no trouble spotting the planet’s signature rings.